In a dramatic showdown, Mia, Max and the synths go into battle with humans; Laura, Niska and Mattie face monumental and life-changing decisions.
Max must confront a friend turned foe. Laura reels from her heartbreaking decision, but when Mattie reveals her secret, she is spurred back into action. Niska’s journey takes an unexpected turn, while Leo races to be by Max’s side, but walks into danger.
Stanley and Anatole’s true colours put Laura and her family directly in harm’s way. With Mia speaking at the commission, Laura must do everything in her power to stop an awful attack. Leo and Mattie return to the Elster house where Leo comes to understand a painful truth, but Mattie has a secret that could knock their fledging relationship off course. Niska continues her mysterious journey that takes her to Wales, where she must defy all her instincts to survive.
While Sam reels from the events of Waltringham and Joe struggles to help him through it, truths are revealed and dangerous plans put into action at the Railyard.
Laura leads a tense Dryden Commission on a visit to the Railyard, but will Agnes’ anger threaten its success? Mia comes face to face with Ed forcing her to confront her past, while an evening out for Karen, Sam and Joe turns to tragedy.
As Leo struggles to come to terms with being human, Mattie takes him back to the Hawkins’ home, where a surprise night out allows them to confront their feelings for each other. Laura is determined to uncover the truth about the too-secret Operation Basswood and tries to enlist Dr Neil Sommer’s help, only for things to take an unexpected turn. Joe reluctantly decides to help Karen with Sam, and Niska receives a mysterious message.
A grieving Max must face the consequences of his painful decision. Laura discovers she’s not being told everything at the Dryden Commission, while Joe uncovers Karen’s shocking secret.
When fractures within the Synth community start to appear, all hope of stability comes under threat, and both families are brought back to fight for the Green-Eyes’ survival.
The acclaimed drama concludes. In the aftermath of the Silo tragedy Karen feels she and Sam have no future and takes matters into her own hands. Niska and Astrid prepare to return to Berlin, but can Niska really walk away from her family? Joe leaves Laura shell-shocked with a frank admission. Hester seeks revenge and will stop at nothing to hurt Leo.
When Mattie reveals that the Synths are planning to break into the Silo, Laura is determined to find Niska and ask for her help. Mattie asks Max to help save Leo from Hester’s influence. Toby receives a disturbing message from Renie. Karen seeks out Athena’s expertise and makes a life-altering decision. Mia and Hester break into the Silo, but things don’t go according to plan.